A light emitting diode (LED) is a compact semiconductor device that generates light of various colors when a current is passed through it. The color depends primarily upon the chemical composition of the light emitting components of the LED die. LEDs exhibit various advantages over incandescent, fluorescent, and discharge light sources, including smaller size, longer life, lower power requirements, good initial drive characteristics, high resistance to vibration and high tolerance to repeated power switching. Because of these favorable characteristics LEDs are widely used in such applications as indicators and low-power lighting applications.
Recently red, green and blue (“RGB”) LEDs having high luminance and efficiencies have been developed and employed as pixel elements in large screen LED displays and signs. This type of LED display can be operated with less power consumption than the prior art, such as incandescent lamps, and has additional favorable characteristics such as light weight and long life. Demand for LEDs as an alternative to prior art display pixel elements is burgeoning.
Although LEDs are more efficient than prior art light sources, they are not 100% efficient in converting electrical energy to light. As a result, a great deal of heat can be produced by the LED die. If the heat is not adequately dissipated, mechanical stress is imposed on various internal components of the LED due to the differing coefficients of thermal expansion of the internal components. This stress can lead to failure of the LED. Therefore, heat sinks are often employed to dissipate heat generated by the LED. The heat sink is usually provided through the metal leadframe of the LED.
The amount of heat generated by the LED becomes an even greater concern as higher-power LEDs are developed for high-brightness applications. Some manufacturers have produced more powerful LEDs having large heat sinks but at a trade-off. First, if an LED with a large heat sink is soldered using conventional methods (i.e. wave solder, reflow solder), the heat from the soldering process is transferred to the LED die, which may cause failure of the LED. Second, if the LED is soldered using non-conventional techniques (i.e. bar soldering or laser soldering), this must generally be performed by the LED manufacturer due to the heat sensitive nature of the process. Therefore, the LED manufacturer often provides a high power LED as a prepackaged component. Unfortunately, the configuration of the package may not be compatible with the physical space requirements of the intended end product design.
In addition, optical coupling of the LED to an associated lens is inefficient. Generally, an LED consists of a semiconductor die adhered to a substrate using an optically clear epoxy. This direct interface of the die (which has a typical index of refraction “n” of about 3.40) to the epoxy (having a typical index of refraction “n” of about 1.56) creates a significant index of refraction gradient between the two materials. As light travels from a medium of high index of refraction to low index of refraction, Fresnel losses are experienced due to the inability of the light to escape the package as a result of internal reflection. Therefore, a material or a layer of material that minimizes the index of refraction gradient is desired to decrease the Fresnel losses that would otherwise occur.
Furthermore, because the epoxy used to encapsulate the conventional LED die is generally rigid when fully cured, thermal expansion of the LED components can cause a degree of shear and tensile stress on the bonds between the bonding wires that connect between the electrical contacts and the LED die. A means of reducing stress on the bonding wires as a result of thermal expansion of the LED components is needed.
Finally, when incorporated into various product applications, LEDs (in their numerous package designs) are generally designed to be assembled onto a printed circuit board (“PCB”) and secured using a soldering process. However, an LED package that can be assembled into products using mechanical processes, such as pin-and-socket arrangements, is desired so that the LED is more adaptable for automated manufacturing processes, consumes less PCB space than previously required, and can accommodate a wider variety of product designs. A mechanical means to install LEDs into a product rather than soldering is also needed to reduce the amount of heat to which the LED die is exposed, thereby eliminating a significant source of LED failure. In addition, an LED is needed that can provide substantially greater heat sinking than that provided by conventional LEDs coupled to a printed circuit board.